Hard-disk drives have rotating high precision disks that are coated on both sides with a special thin film media designed to store information in the form of magnetic patterns. Electromagnetic read/write heads suspended or floating only fractions of micro inches above the disk are used to either record information onto the thin film media, or read information from it.
A read/write head may write information to the disk by creating an electromagnetic field to orient a cluster of magnetic grains in one direction or the other. Each grain will be a magnetic dipole pointing in a certain direction and also creating a magnetic field around the grain. All of the grains in a magnetic region typically point in the same direction so that the magnetic region as a whole has an associated magnetic field. The read/write head writes regions of positive and negative magnetic polarity, and the timing of the boundaries between regions of opposite polarity (referred to as “magnetic transitions”) is used to encode the data. To increase the capacity of disk drives, manufacturers are continually striving to reduce the size of the grains.
The ability of individual magnetic grains to be magnetized in one direction or the other, however, poses problems where grains are extremely small. The superparamagnetic effect results when the product of a grain's volume (V) and its anisotropy energy (Ku) fall below a certain value such that the magnetization of that grain may flip spontaneously due to thermal excitations. Where this occurs, data stored on the disk is corrupted. Thus, while it is desirable to make smaller grains to support higher density recording with less noise, grain miniaturization is inherently limited by the superparamagnetic effect.
Perpendicular recording addresses this “thermal” limit. In conventional “longitudinal” magnetic recording, the magnetization in the bits is directed circumferentially along the track direction. In perpendicular recording, the magnetic bits point up or down perpendicular to the disk surface.
Granular magnetic films such as CoCrPt-MOx used in modern perpendicular magnetic data storage rely on one or more segregants for grain isolation. The segregant, denoted as M in the above formula, is a material with low surface energy and low affinity. During the sputtering process of the magnetic film, the low-surface-energy segregant comes out of the sputtering solution and moves toward the grain boundary to form a boundary area for each grain.
However, due to the nature of random nucleation, the locations of magnetic grains are also random. This leads to the formation of a zig-zag boundary between data bits, which may cause difficulties in forming tracks around the disk. Another issue is the distribution of grain size. To maximize data density, grain size needs to be uniform and as small as possible. However, with conventional processes, non-uniformity in grain size may occur, which can degrade recording performance. Finally, random nucleation also leads to random grain boundary thickness and random average grain boundary thickness distribution.